Solid dielectric capacitors are well-known and commonly used in electrical circuits. In many instances, fused capacitors are desired such that excessive current will be interrupted by the fuse in the capacitor and prevent damage to other components in the circuit. With respect to solid electrolyte capacitors such as tantalum capacitors, the capacitors have a defined polarity, i.e., defined anodes and cathodes. If these solid electrolyte capacitors are installed in a circuit backwards, the capacitor will not function properly. Consequently, fusing can prevent damage to the electrical circuit caused by the incorrect installation of the capacitor. Moreover, with tantalum capacitors, fusing is desired in applications where high power levels are present, since a short circuit can possibly result in enough heat being generated to ignite the tantalum metal which burns exothermically.
Fused capacitor assemblies are disclosed, for instance, in U.S. Pat. Nos. 4,106,184; 4,107,762; 4,224,656; and 4,763,228, and in Japanese laid-open patent applications (Kokai) 62-272156 and 63-128707. In one type of capacitor, the fuse extends between terminals inside the encapsulating insulator of the capacitor such that all electrical
2 current current passing between the terminals passes through the fuse.
Numerous advances in fuse materials have occurred. Exothermic fuses are generally preferred and widely used. Unlike conventional fuses that are adapted to melt at a predetermined current level, exothermic fuses contain metals which alloy exothermically when brought to a threshold temperature. The exothermic alloying breaks electrical continuity. Typical exothermic fuses comprise a bimetallic composite of aluminum and a precious metal such as palladium. Su, in U.S. Pat. No. 4,763,228, discloses a further improvement in exothermic fuse assemblies in which the fuse material is surrounded by silicone such that when the fuse ignites, an electrically-conductive carbon residue does not result which could otherwise serve to conduct electricity between the terminals.
Particular attention has been directed by workers in the field to the manufacture of capacitors (particularly electrolytic capacitors) containing fuses, especially exothermic fuses, and the design of the capacitors to assure reliable performance of the fuses. Capacitor assembly, in order to be competitive, must be capable of being automated. To facilitate automation, lead frames containing the electrode leads for a plurality of capacitors are used and the capacitor is built on these frames. The frames can be easily transported from station to station. The use of these frames is virtually mandated by the miniaturization of capacitors which can be too small to easily handle on a capacitor-by-capacitor basis.
The lead frames should therefore be designed to enable all assembly operations to be automated. Automating the installation of the fuse wire is particularly difficult in that the fuse wire is delicate, e.g., may only be 0.002 inch in diameter, and, if an exothermic fuse is used, risks of igniting the fuse during installation exist. The ease of fabrication, however, cannot be at the expense of performance. For example, when assembling circuit boards, the components such as capacitors are typically inserted in the boards and then the boards are bathed in solder. Any solder bridging that may occur between the terminals bridging the fuse would defeat the purpose of the fuse.
Japanese Kokai 62-272516, Nov. 26, 1987, discloses a fused, electrolytic capacitor in which a lead frame has a cathode lead (negative lead) and either a "U"-shaped or "L"-shaped lead positioned between the anode lead (positive lead) and the electrolytic capacitor body (i.e., the component of the capacitor performing the electronic function). A wire extends from the electrolytic capacitor body to the either "U"-shaped or "L" shaped lead and a fuse extends between that lead and the anode lead. This type of capacitor design can provide several problems. First, both the "U"-shaped (or "L"-shaped) lead and the anode lead extend from the insulating casing in close proximity. This increases the risk of solder bridging. Second, especially with the "L"-shaped lead, it is relatively easy for this lead and the anode lead to move with respect to each other making automated assembly more difficult. Further, severing the "U"-shaped or "L"-shaped lead from the lead frames poses difficulties especially in view of its proximity to the anode lead.
Japanese Kokai 63-128707, June 1, 1988, discloses another electrolytic capacitor design. In making this capacitor, a lead frame is used which has a plurality of anode leads and a plate, or ribbon, perpendicular to the anode leads which join each of the anode leads at the ends intended to be encapsulated in insulator material. The ribbon has elongated slots such that the sides of the ribbon are joined only between the capacitors when assembled. The anode lead wire from the electrolytic capacitor body is attached to one side of the ribbon. A fuse is installed to provide electrical communication between the sides of the ribbons. After assembly of the fuse, the capacitor is severed. A cutting block is used in severing the ribbon or more exotic means such as laser cutting is used, either of which increase the complexity of automated assembly. When using a cutting block, the capacitor is placed on the block and a blade is used to sever the ribbon on each side of the encapsulated capacitor. While this design provides greater stability than that described above and severing the ribbon is facilitated, there remain drawbacks. For instance, the close proximity of each side of the ribbon as it protrudes from the encapsulating insulator provides significant risk of solder bridging.